(1) Field of the Invention
The invention relates to the field of sonar measuring devices and in particular to devices for measuring the speed of sound in water.
(2) Description of the Prior Art
The use of sonar systems continues to broaden, covering depth finding, target detection, location, ranging and all traditional uses. New uses include the aiming of small underwater projectiles against relatively small incoming threats, e.g., underwater bullets fired at incoming torpedoes. Each of these new uses requires increased resolution, speed, and accuracy from a sonar tracking or aiming system. In order to provide this increased resolution, speed and accuracy, it is important to minimize errors in the measurement of signals from these threats. Significant errors can result from inaccurate sound velocity profiles in the water column. Currently the U.S. Navy uses expendable bathythermometric (XBT) devices to measure the temperature profile of the water column. From this measured temperature profile, the sound speed profile is calculated and used to improve sonar performance. These XBT devices have a number of drawbacks including the inability to measure the sound speed directly, the lack of continuous measurement, the requirement for repeated deployment intervals during a day, and the considerable debris generated with intensive use. With the rapidly changing conditions found in shallow, littoral water environments (environments having fresh water influxes), the need for a realtime continuous sound speed measurement is crucial to maintain optimum sonar performance. In these types of environments, XBT's are least effective as not only the water temperature is changing rapidly, but the salinity is also changing. Since XBT's measure the speed of sound indirectly (based on temperature), significant inaccuracies can result. Additionally, the rapidly changing conditions require even greater numbers of XBT deployments.
Other examples of prior art devices for measuring sound speed include direct measurement devices for static, non-moving systems and indirect, calculated determinations for moving systems. For example, U.S. Pat. No. 4,558,437 to Meeder et al. disclosed a static device having an emitter at the seafloor with a receive sensor located at the boat. Meeder et al. disclose a static system which measures sound pressure levels in ocean sediment, but cannot be adapted to measurement throughout the water column, nor to moving systems. Typically, moving systems depend on calculation and/or indirect measurements. U.S. Pat. No. 5,608,689 to Capell, Sr. is an example disclosing a moving system which calculates the speed of sound based on transmitting and receiving at one location (on the ship). The necessary calculations, based on multiple pulses, require a precise knowledge of transmission and reception angles.
What is needed is a realtime means for directly measuring the speed of sound profile throughout a water column. Additional measurement from a moving platform is required and measurement using a simple omni-directional sound source is preferable.